Propylene polymers and process for the preparation thereof

ABSTRACT

A process for preparing propylene polymer comprising contacting, under polymerisation conditions, propylene and optionally ethylene, with a catalyst system comprising a bridged metallocene and a suitable activating cocatalyst, said process being characteised by reducing the concentration of hydrogen formed during the polymerization reaction.

[0001] The present invention relates to propylene polymers, optionallycontaining up to 5 mol % of ethylene, having a number of internalvinylidene unsaturations per polymer chain greater than or equal to 2;these polymers are obtained by removing at least a part of hydrogenpresent in the polymerization apparatus during the process.

[0002] It is known that polypropylene, although possessing goodphysicomechanical properties and excellent chemical resistance, lackhighly desirable properties, such as varnishability, dyeing adhesion andcompatibility with other polymers or inorganic substrates because of theapolar and saturated nature. Introducing a high number of unsaturationsin the polymer chain therefore could be a way for obtaining afunctionalizable polymer and for eliminating these disadvantages. It isknown that the polymerization of olefins carried out with metallocenecomplexes gives rise to evolution of molecular hydrogen. For instance J.Am. Chem. Soc. 1998, 120, 2174-2175 shows that gas-phase reactionsbetween ethylene or an alpha-olefil and Cp₂ZrCH₃ ⁺ during massspectroscopic studies result in elimination of molecular hydrogen withconcomitant formation of an eta-3-allyl complex. This evolution has beenassociated with the formation of unsaturations in the polymer chain.Formation of internal unsaturations in the polymer chain inethylene/alpha-olefin copolymers produced with metallocenes is reportedin Polymers Preprints 1998, 39(2), 425. However these documents do notrelate to propylene polymerization and moreover, in these documents,there is no indication that the nature and the number of internalunsaturation in a propylene polymer may be controlled. Chain transferreactions in polypropylene polymerization have been investigated byRescomi at al. in Topics in Catalyst 1999, 7, 145-163, but he does notrelate to hydrogen produced during the polymerization reactions.

[0003] EP 778293 relates to a process for producing an olefin polymerwhere an olefin is polymerized in the presence of a metallocene complex;by forced removing hydrogen during the polymerization process, it ispossible to obtain an olefin polymer having a desired melt index. Inthis document the presence of hydrogen in the process is explained bythe formation of unsaturated bonds at the terminal end of olefinpolymer, no reference being made to internal unsaturations; moreoveronly ethylenel-hexene is polymerized in the examples. The Applicant hasnow unexpectedly found a new propylene polymer, optionally containing upto 5 mol % of ethylene, having the following characteristics:

[0004] i) molecular weight distribution (M_(W)/M_(n))≦4;

[0005] ii) number of internal vinylidene per polymer chain ≧2.

[0006] The propylene polymer object of the present invention isobtainable with a polymerization process carried out in the presence ofa metallocene-based catalyst system, by selectively removing at leastpart of hydrogen present in the polymerization apparatus during thepolymerization process. More specifically, it is another object of thepresent invention a process for preparing the above described propylenepolymers comprising contacting, under polymerization conditions,propylene and optionally ethylene, with a catalyst system comprising:

[0007] a) a metaltocene complex of formula (I)

[0008]  wherein:

[0009] M is titanium zirconium or hafnium; preferably M is zirconium;

[0010] the groups X equal to or different from each other, aremonoanionic sigma ligands selected from the group consisting ofhydrogen, halogen, —R, —OR, —OCOR, —OSO₂CF₃, —SR, —NR₂ and —PR₂, whereinR is a linear or branched C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl C₃-C₂₀cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀ alkylaryl or C₇-C₂₀ arylalkyl radical;preferably R is methyl, ethyl, propyl, butyl or phenyl; preferably X ishalogen or C₁-C₂₀ alkyl;

[0011] the groups R¹, R², R³ and R⁴, equal to or different from eachother, are selected from the group consisting of hydrogen, linear orbranched C₁-C₂₀ akyl, C₂-C₂₀ alkenyl, C₃-C₂₀ cycloalkyl, C6-C20 aryl,C₇-C₂₀ alkylaryl or C₇-C₂₀ arylalkyl radicals, optionally containingheteroatoms belonging to groups 13-17 of the Periodic Table; two or fouradjacent groups R¹, R², R³ and R⁴ may form together one or more 3-6membered aromatic or aliphatic rings, optionally substituted withhydrocarbyl radicals optionally containing heteroatoms belonging togroups 13-17 of the Periodic Table; preferably R¹, R², R³ and R⁴ arehydrogen or C₁-C₂₀ alkyl; optionally containing nitrogen, phosphorus orsuifir, or R¹ and R² form a six-memnbered aromatic or aliphatic ring;

[0012] with the proviso that either R¹ is different from R⁴ or R² isdifferent from R³;

[0013] Z is a carbon or silicon atom; preferably Z is a carbon atom;

[0014] the groups R¹ and R⁶, equal to or different from each other, areselected from the group consisting of hydrogen, linear or branchedC₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ cycloalkyl, C6-C20 aryl, C₇-C₂₀alkylaryl and C₇-C₂₀ arylalkyl radical optionally containing heteroatomsbelonging to groups 13-17 of the Periodic Table; R⁵ and R⁶ optionallyform together a 3 to 6-membered ring; preferably R⁵ and R⁶ are selectedfrom the group consisting of hydrogen, methyl, ethyl, propyl and phenyl;and

[0015] b) a suitable activating cocatalyst;

[0016] said process being characterized by reducing the concentration ofhydrogen formed during the polymerization reaction

DETAILED DESCRIPTION OF THE INVENTION

[0017] The number of internal vinylidenes per polymer chain is definedas the number of internal vinylidene bonds over the total number ofunsaturated end groups. More precisely, the number of internalvinylidenes per polymer chain is the ratio between the number of bondsin a polymer chain having the following structure:

[0018] over the total number of unsaturated end groups in a polymerchain, having the following stuctures:

[0019] Using N.M.R. techniques, the skilled man in the art can carry outthe analysis of the polymer in order to determine the content ofinternal vinylidene per polymer chain. Examples of N.M.R. assignmentscan be found in Topics in Catalysis 1999, 7, 145 and Journal ofMolecular Catalysis 1999, 146, 167.

[0020] The propylene polymer, optionally containing up to 5 mol % ofethylene, object of the present invention has the followingcharacteristics:

[0021] (i) molecular weight distribution (M_(W)/M_(n) ) ≦4, preferably≦3,

[0022]  more preferably ≦2.5;

[0023] (ii) number. of internal vinylidene per polymer chain ≧2,preferably ≧2.5, more preferably ≧3.

[0024] Moreover, according to a preferred embodiment, the propylenepolymers of the invention have the following characteristic:

[0025] iii) the isotactic pentads (mmmm), as determined by ¹³C-NNManalyses, are ≧80%.

[0026] According to another preferred embodiment, the propylene polymerof the invention, have the following characteristic:

[0027] iv) less than 0.5% o f the CH₂ groups in the polymeric chain arein sequences (CH₂)_(n) wherein n is an even number. The structure of thepropylene polymer according to the invention appears to be highlyregioregular. In fact, from the ¹³C-N.M.R. analysis (125.7 Mz) nosignals are revealed as deriving from the (CH₂)_(n) sequence where n isan even number.

[0028] Preferably the propylene polymers, object of the presentinvention, are obtainable by a process comprising contacting, underpolymerization conditions, propylene and optionally ethylene with acatalyst system comprising:

[0029] a) a metallocene complex of formula (II)

[0030]  wherein

[0031] M, X, R³, R⁴, R⁵ and R⁶ have the meaning reported above and thegroups R⁷, R⁸, R⁹ and R¹⁰, equal to or different from each other, areselected from the group consisting of hydrogen, linear or branchedC₁-C₂₀ alkyl, C₂-C₂₀ akenyl, C₃-C₂₀ cyclolkyl, C₆-C₂₀ aryl, C₇-C₂₀alkylaryl and C₇-C₂₀ arylalkyl radicals, optionally containinghoteroatoms belonging to groups 13-17 of the Periodic Table; preferablyR⁷, R⁸, R⁹ and R¹⁰ are hydrogen, C₁-C₂₀ alkyl or C₆-C₂₀ aryl;

[0032] preferably R³ is a group SiR₃ or CR₃, wherein R has the meaningreported above, R⁴ is hydrogen; more preferably R³ is Si(CH₃)₃ orC(CH₃)₃; and

[0033] b) a suitable activating cocatalyst;

[0034] said process being, characterized in that the concentration ofthe hydrogen formed during the polymerization reaction is reduced.

[0035] Metallocene complexes can be obtained with various processesknown in the art such as, for example, as described in WO 96/22995 andWO 98/43989.

[0036] In the process of the invention, hydrogen can be suitably removedduring the polymerization reaction by means of methods known in the art,such as:

[0037] 1) by using a hydrogenation catalyst in the gas phase, able tocatalytically hydrogenate olefins;

[0038] 2) by using a hydrogenation catalyst in the liquid phase, able tocatalytically hydrogenate olefins;

[0039] 3) by physically removing hydrogen from the gas phase.

[0040] According to methods 1) and 2), hydrogen present in thepolymerization reactor reacts with propylene monomer to produce propane.As a result, the concentration of hydrogen gas in the reaction system isdecreased At this time, the amount of the propane produced is small andit has substantially no adverse effect on the polymerization ofpropylene.

[0041] Examples of hydrogenation catalysts fit for method 1) areplatinum- or palladium-based compositions, particularly preferred isplatinum or palladium on alumina.

[0042] Preferably these catalysts are installed in the gas cap of thereactor.

[0043] Examples of hydrogenation catalysts fit for method 2) are cobalt-or nickel-based catalysts activated by trialylaluminiums, such ascobalt(acetylacetonate) or nickel(octanoate); rhodium catalysts such asWilkinson's catalyst (Rh(PPh₃)₃Cl); ruthenium catalysts, such asRu(H)Cl(PPh₃)₃. Alternatively heterogeneous platinum, platinum oxide orpalladium catalysts may be used as a suspension in the reaction medium.

[0044] Hydrogenation catalysts which can be used for methods 1) and 2)are described in “Catalytic Hydrogenation” (R. L. Augustine, publisherDekker, New York, 1965) and in “Advanced Organic Chemistry”, 4^(th)Edition, p. 771-780 (J. March, publisher Wiley, New York, 1992).

[0045] Once selected the hydrogenation catalyst, those skilled in theart can select the necessary amount of it depending on the catalystactivity, according to common procedures.

[0046] According to method 3), hydrogen can be physically removed fromthe reactor by using, for example, a solid or a liquid adsorbent whichcan selectively adsorb hydrogen or a hydrogen separating membrane thatallows hydrogen to permeate; alternatively the gas cap of the reactorcan be vented. The hydrogen concentration during the polymerizationreaction, is reduced to less than 50%, preferably to less than 30% andmore preferably to less than 20% of the hydrogen concentration inabsence of means to reduce it (hydrogenation catalysts or physicalmeans).

[0047] In other words, the hydrogen concentration in the reactor in thepresence of hydrogenation means has to be less than 50% preferably lessthan 30% more preferably less than 20% of the hydrogen concentrationthat would be in the reactor under about the same reaction conditionsbut in absence of means for removing hydrogen. When the polymerizationreaction is carried out in liquid phase, hydrogen concentration in thegas phase of the reactor (i.e. in the gas cap) can vary from 0 to 0.020%mol. more preferably from 0 to 0.015% mol.

[0048] Suitable activating cocatalysts according to the process of theinvention are alumoxanes or compounds capable of forming an alkylmetallocene cation.

[0049] Alumoxane useful as cocatalyst (b) may be linear alumoxanes ofthe formula (III):

[0050] wherein R¹¹ is selected from the group consisting of halogen,linear or branched, saturated or unsaturated C₁-C₂₀ alkyl, C₃-C₂₀cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀ alkylaryl and C₇-C₂₀ arylalkyl radicalsand y ranges from 0 to 40;

[0051] or cyclic alumoxanes of the formula (V):

[0052] wherein R¹¹ has the meaning herein described and y is an integerranging from 2 to 40.

[0053] The above alumoxanes may be obtained according to proceduresknown in the state of the art, by reacting water with an organo-aluminumcompound of formula AlR¹¹ ₃ or Al₂R¹¹ ₆, with the condition that atleast one R¹¹ is not halogen. In this case, the molar ratios of Al/waterin the reaction is comprised between 1:1 and 100:1. Particularlysuitable are the organometallic aluminum compounds described in formula(H) of EP 0 575 875 and those described in formula (II) of WO 96102580.Moreover, suitable cocatalysts are those described in WO 99/21899 and inthe European app. no. 99203110.4.

[0054] The molar ratio between aluminum and the metal of the metallocenecomplex is comprised between about 10:1 and about 20000:1, preferablybetween about 100:1 and about 10000:1, more preferably between 100:1 andabout 5000:1.

[0055] Examples of alumoxanes suitable as activating cocatalysts in theprocess of the invention are methylalumoxane (WO),tetra-isobutyl-alumoxane (CIAO), tetra-2,4,4-trimethylpentyl-alumoxane(MIOAO) and tetra-2-methyl-pentylalumoxane. Mixtures of differentalumoxanes can also be used. Not limiting examples of aluminum compoundsof formula AlR¹¹ ₃ or Al₂R¹¹ ₆ are:

[0056] tris(methyl)aluminum, tris(isobutyl)aluminum,tris(isooctyl)aluminum,

[0057] methyl-bis(isobutyl)aluminum, dimethyl(isobutyl)aluminum,tris(isohexyl)aluminum,

[0058] tris(benzyl)aluminum, tris(tolyl)aluminum,tris(2,4,4trimethylpentyl)aluminum,

[0059] bis(2,4,4-trimethylpentyl)aluminum hydride,isobutyl-bis(2-phenyl-propyl)aluminum,

[0060] diisobutyl-(2-phenyl-propyl)aluminum,isobutyl-bis(2,4,4-trimethyl-pentyl)aluminum,

[0061] diisobutyl-(2,4,4trimethyl-pentyl)aluminum,tris(2,3-dimethyl-hexyl)aluminum,

[0062] tris(2,3,3-trimethyl-butyl)aluminum,tris(2,3-dimethyl-butyl)aluminum,

[0063] tris(2,3-dimethyl-pentyl)aluminum,tris(2-methyl-3-ethyl-pentyl)aluminum,

[0064] tris(2-ethyl-3-methyl-butyl)aluminum,tris(2-ethyl-3-methyl-pentyl)aluminum,

[0065] tris(2-isopropyl-3-methyl-butyl)aluminum,tris(2,4-dimethyl-heptyl)aluminum.

[0066] tris(2-phenyl-propyl)aluminumtris(2-(4-fluoro-phenyl)-propyl)aluminum and

[0067] tris(2-(4-chloro-phenyl)-propyl)aluminum

[0068] as well as the corresponding compounds where one or more of thehydrocarbyl groups is replaced by a hydrogen atom.

[0069] Particularly preferred aluminum compounds are trimethylaluminum(TMA), tris(2,4,4trimethylpentyl) aluminum (TIOA), triisobutylaluminum(TIBA), tris(2,3,3-trimethyl-butyl)aluminum,tris(2,3-dimethyl-butyl)aluminum, tris(2-phenyl-propyl) aluminum,tris[2-(4-fluoro-phenyl)-propyl]aluminum andtris[2-(4-chloro-phenyl)-propyl]aluminum. Mixtures of differentorganometallic aluminum compounds and/or alumoxanes can also be used. Inthe catalyst system used in the process of the invention, both saidmetallocene complex and said alumoxane can be pre-reacted with anorganometallic aluminum compound of formula AlR¹¹ ₃ or Al₂R¹¹ ₆, whereinR¹¹ has the meaning reported above.

[0070] Further suitable cocatalysts are those compounds capable offorming an alkylmetallocene cation; preferably, said compounds haveformula Y⁺D⁻, wherein Y⁺ is a Brønsted acid capable of donating a protonand of reacting irreversibly with a substituent X of the compound offormula (I) or (II), and D⁻ is a compatible non-coordinating anion,capable of stabilizing the active catalytic species which result fromthe reaction of the two compounds, and which is sufficiently labile tobe displaceable by an olefinic substrate. Preferably, the D⁻ anioncomprises one or more boron atoms. More preferably, the anion D⁻ is ananion of formula BAr₄ ⁽⁻⁾, wherein the Ar substituents, the same ordifferent from each other, are aryl radicals such as phenyl,pentafluorophenyl, bis(triuoromcthyl)phenyl.Tetrakis-pentafluorophenyl-borate is particularly preferred. Moreover,compounds of formula BAr₃ can be conveniently used. The process forobtaining the propylene polymer of the present invention can also becarried out by depositing the metallocene complex of formula (I) or (II)or the reaction product of the the metallocene complex of formula (I) or(II) with a suitable cocatalyst, or the suitable cocatalyst andsuccessively the metallocene complex of formula (I) or (II), on theinert support, such as silica, alumina, magnesium halides, olefinpolymers or prepolymers (i.e. polyethylenes, polypropylenes orstyrene-divinylbenzene copolymers). The thus obtained supported catalystsystem, optionally in the presence of alkylaluminum compounds, eitheruntreated or pre-reacted with water, can be usefully employed ingas-phase polymerization processes. The solid compound so obtained, incombination with further addition of the alkyl aluminum compound as suchor prereacted with water, is usefully employed in gas phasepolymerization. The polymerization process according to the presentinvention can be carried out in gaseous phase or in liquid phase,optionally in the presence of an inert hydrocarbon solvent eitheraromatic (such as toluene), or aliphatic (such as propane, hexane,heptane, isobutane and cyclohexane).

[0071] The polymerization temperature ranges from about 0° C. to about250° C., preferably from 20° C. to 150° C., and more preferably from 40°C. to 90° C.

[0072] The molecular weight distribution can be varied by using mixturesof different metaflocenes or by carrying out the polymerization invarious steps differing in the polymerization temperature and/or in theconcentration of the polymerization monomers.

[0073] Metallocene complex of formula (I) or (II) and the suitablecocatalyst may be contacted among them before the polymerization Thecontact time may be comprised between 1 and 60 minutes, preferablybetween 5 and 20 minutes. The pre-contact concentrations for metallocenecomplex of formula (I) or (II) are comprised between 10⁻² and 10⁻⁸ moll,whereas for the suitable cocatalyst they are comprised between 10 and10⁻³ mol/l. The precontact is generally carried out in the presence of ahydrocarbon solvent and, optionally, of small amounts of monomer. Thepropylene polymers of the present invention can be crosslinked either byelectron beam irradiation or with chemical reagents. The propylenepolymers of the present invention are suitable for various kinds ofapplications, such as insulated wire and other electric parts, printedcircuit boards, heat insulating materials, packaging materials androofing materials, by introduction of functional groups by chemicalmeans into the hydrocarbon structure for further advanced applications.A further advantage of the propylene polymers of the present inventionis that the internal vinylidene unsaturations are not subjected tothermal degradation or oxidative degradation if compared withpolyolefins having unsaturations on the backbone. The following examplesare given for illustrative purposes and are not intended to limit thescope and spirit of the invention.

EXAMPLES

[0074] General Procedures and Characterizations:

[0075] Methylaluminoxane (MAO) was purchased from Witco as a 10% w/wsolution in toluene. IThe metallocenes rac-C₂H₄(1-Ind)₂ZrCl₂ andrac-C₂H₄(4,5,6,7-tetrahydro-1-Ind)₂ZrCl₂ were synthesized according toEP-A-575,875. The metallocene rac-Me₂C(3-SiMe₃-Ind)₂ZrCl₂ wassynthesized according to WO 96/22995. According to ¹H-NMR spectroscopicmeasurements, the metallocenes were effectively pure rac isomers.

[0076] Toluene (Aldrich, anhydrous grade) was stripped with nitrogenovernight and dried with 4 Å molecular sieves (H₂O level˜5-8 ppm).Nitrogen and propylene (liquid, 12 bar) were purified over a BTS columncontaining a copper catalyst (BASF R3-11) as well as a column containinga mixed bed of molecular sieves (3 Å (bottom), 4 Å (mid part) and 13×(top)) to remove oxygen and water respectively (O₂ level˜0.3 ppm, H₂Olevel 0.5 ppm). Nitrogen and propylene feedstrears were continuouslymonitored at ppm level. AMS and Systech analyzers were used to determinethe oxygen content. MCM analyzers were used to determine the watercontent. RGA3 (reduction gas analyzer; Trace Analytical) was used todetermine the CO content (ppb range).

[0077] Gas cap composition during polymerizations was analyzed by meansof an Orbisphere hydrogen analyzer (hydrogen gas cap composition at 15sintervals) and also using a GC-apparatus from Intersciencepropane/propylene ratio at 1 minute intervals). The GC determinationswere performed using a GC 8000 chromatograph with a wide bore(Al₂O₃/KCl) capillary column under isothermal conditions. The componentswere detected by means of a flame ionization detector (FID) and/orthermal conductivity detector (TCD).

[0078] Gel Permeation Chromatography (GPC) Analysis

[0079] High-temperature GPC analyses were carried out using a Waters 150CV instrument. A single solution of each sample was prepared by adding15 ml of solvent to ca. 30 mg of sample and refluxing gently for 20minutes. The solutions were then filtered through a fiber pad at 140° C.and part of each filtered solution transferred into special glass samplevials. The vials were then placed in a heated sample compartment andafter an initial delay of 20 minutes to allow the samples to equilibratethermally, injection of part of the contents of each vial was carriedout automatically in series.

[0080] The following chromatographic conditions were used: Column: PLgel2 × mixed bed-B, 30 cm, 10 microns Solvent: 1,2-dichlorobenzene withantioxidant Flow rate: 1.0 ml/minutes Temperature: 140° C. Detector:refractive index Calibration: polystyrene

[0081] NMR Characterization

[0082]¹H and ¹³C NMR spectra of the propylene polymers were recorded in1,2,4trichlorobenzene or 1,2-dichlorobenzene solution at 130° C. (500 Mfor ¹H and 125.7 MHz for ¹³C NMR). NMR data are listed in parts permillion downfield from MS for proton and carbon, residual hydrogenresonances in the solvent being used for reference. The degree ofregiotegularity (sum of threo- and erythro-2,1-insertions and1,3-insertions) and stereospecificity (mmmm) were determined by ¹³C NMRspectroscopy using standard procedures, as detailed in Macromolecules1995, 28, 6667 and references therein.

[0083] The relative amounts of the different unsaturated end groups andinternal vinylidene group (vin II) per chain were determined by ¹H NMRspectroscopy. The total number of unsaturated end groups per chain wasnormalized to 1.0. The number of vin II groups per chain was determinedas the ratio of the vin If groups to the total unsaturated end groups.The assignments were determined as described in Topics in Catalysis1999, 7, 145 and in Journal of Molecular Catalysis 1999, 146, 167.

[0084] The following assignments are used (¹H NMR, solvent 1,2-C₂D₄C₁₂,temperature 130° C., internal reference TMS).

δ 4.82

δ 4.78, δ 4.72

δ 5.85, δ 5.01

δ 5.50

δ 5.25

[0085] Polymerization Procedure

[0086] Contact of the above mentioned materials with water and oxygenwas minimized by conducting the chemical reactions in a dry nitrogenatmosphere in a glovebox, under Schlenk conditions or in bottlesprovided with a septum cap. Batch polymerization reactions were carriedout in a 5 liters reactor provided with a turbine stirrer. A steam/watersystem was used for temperature control. Exotherms were monitored bydetermining (thermocouple) the temperature difference between thereactor contents and the incoming cooling water.

EXAMPLE 1

[0087] This example illustrates the application of a supported platinumhydrogenation catalyst, which was installed in the gas cap of thereactor. The hydrogenation catalyst consisted of pellets of 1% Pt onalumina (ca. 20 g) enveloped in a stainless steel gauze, which wasmounted just underneath the lid of the reactor to prevent contact withthe liquid phase.

[0088] The 5 liter reactor was heated to 150-160° C. overnight, whilstpurging with nitrogen, cooled and then pickled at 70° C. using a mixtureof triisobutyl aluminum (TIBA, 0.25 g), toluene (20 ml) and propylene(500 g). The pickle mixture was removed and the reactor then chargedwith 1650 g liquid propylene, whilst increasing the temperature from 20°C. to 50° C. A solution of MAO in toluene (4.74 g of 4.95% wt solution;8.69 mmol) was then introduced into the reactor using an injectionsystem and washed in using 20 ml of toluene.

[0089] Meanwhile, rac-Me₂Si(3-SiMe₃-Ind)₂ZrCl₂ (13.1 mg, 22.7 timol) wasdissolved in toluene (12.5 g), and the obtained solution was reactedwith MAO (6.20 g of 4.95% wt solution). 10 minutes after theintroduction of the hydrolyzed alkylaluminium mixture into the reactor,0.411 g of the catalyst solution (aged for 5 minutes; 0.50 μmol) wasinjected into the reactor (using 20 ml toluene) and the polymerizationwas continued using 840-1100 rpm stirring. The hydrogen concentrationincreased within ca. 15 minutes to a constant level of 0.004% mol (gascap). The polymerization was stopped after 300 minutes by injection of5-10 ml methanol. The heating was then discontinued and the propylenerapidly vented and the powder polypropylene collected. The polypropylenewas dried (70-80° C., 200 mbar, nitrogen purge) giving the yield ofproduct

[0090] The polymerization conditions are reported in table 1, while thedata relating to the obtained polymers are indicated in Table 2.

EXAMPLE 2

[0091] Example 2 was performed similarly to Example 1, but with a highercatalyst intake. The catalyst and cocatalyst intake, polymerizationconditions and the data relating to the obtained polymers are summarizedin Tables 1 and 2. The hydrogen concentration increased within ca. 10minutes to a constant level of 0.010% mol (gas cap).

EXAMPLE 3

[0092] Example 3 illustrates the application of a hydrogen flashingprocedure to remove hydrogen from the reactor. The procedure was similarto Example 1, except that the platinum hydrogenation catalyst wasomitted. The catalyst and cocatalyst intake, polymerization conditionsand the data relating to the obtained polymers are summarized in Tables1 and 2. After injection of the catalyst, the gas cap (primarilypropylene) was allowed to vent slowly. The hydrogen concentrationincreased within ca. 3 minutes to a constant level of 0.013% mol (gascap). The polymerization was terminated after 30 minutes using amethanol kill as described in Example 1.

EXAMPLE 4

[0093] Example 4 illustrates the application of a platinum oxidehydrogenation catalyst utilized as a slurry in liquid propylene. Theprocedure was similar to Example 1, except that the gas phase platinumcatalyst was omitted and a toluene slurry of PtO₂ (20 g, containing 220mg of PtO₂) was injected prior to addition of the liquid propylene, andadditional amounts of catalyst (as a slurry in 20 g of toluene) wereadded 35 minutes and 50 minutes after catalyst injection (55 mg and 110mg PtO₂, respectively). The catalyst and cocatalyst intake,polymerization conditions and the data relating to the obtained polymersare summarized in Tables 1 and 2. The hydrogen concentration slowlyincreased during the course of the polymerization to a final level of0.015% mol (in the gas cap).

EXAMPLE 5

[0094] Example 5 was performed similarly to Example 1, but with a lowlevel of added ethylene (0. 12% mol, gas cap). As in Example 1 aPt/alumina catalyst in the gas cap was utilized for hydrogen removal.The catalyst and cocatalyst intake, polymerization conditions and thedata relating to the obtained polymers are summarized in Tables 1 and 2.Hydrogen concentration increased within ca. 10 minutes to a constantlevel of 0.010% mol in the gas cap.

COMPARATIVE EXAMPLE 6

[0095] Comparative Example 6 was performed similarly to Example 1, butwithout removal of hydrogen. The catalyst and cocatalyst intake,polymerization conditions and the data relating to the obtained polymersare summarized in Tables 1 and 2. Hydrogen concentration increasedwithin ca. 25 minutes to a constant level of 0.07% mol in the gas cap.

COMPARATIVE EXAMPLE 7

[0096] Comparative Example 7 was performed similarly to Example 1, butwith rac-C₂H₄(1-Ind)₂ZrCl₂ as catalyst precursor instead ofrac-Me₂Si(3-SiMe₃-Ind)₂ZrCl₂. As in Example 1 a Pt/alumina catalyst inthe gas cap was utilized for hydrogen removal. The polymerizationconditions are reported in Table 1, while the data relating to theobtained polymaers are indicated in Table 2. Hydrogen concentrationincreased within ca. 10 minutes to a constant level of 0.013% mol in thegas cap.

COMPARATIVE EXAMPLE 8

[0097] Comparative Example 8 was performed similarly to Example 1, butwith rac-C₂H₄(4,5,6,7-tetrahydro-1-Ind)₂ZrCl₂ as catalyst precursor andwithout removal of hydrogen. The catalyst and cocatalyst intake,polymerization conditions and the data relating to the obtained polymersare summarized in Tables 1 and 2. TABLE 1 Cocatalyst Metallocene (MAO)Means for Hydrogen in Activity amount Al/Zr Premix (mmol) reducing gascap % hydrogen Yield (Kg PP/g Example Metallocene (μmol) (mol/mol)Reactor (mmol) T ° C. hydrogen % mol, reduced (g. PP) Zr. h) 1 A 0.5017900 0.250 8.69 50 Pt 0.004 90 125 548 2 A 1.00 9000 0.500 8.69 50 Pt0.010 80 68 471 3 A 3.00 3000 1.494 7.50 50 Vented 0.013 80 30 389 4 A2.00 4800 1.000 8.69 50 PtO₂ 0.015 70 73 218  5* A 0.50 17900 0.250 8.6950 Pt 0.010 75 37 749 Comp. 6 A 3.00 3000 1.494 7.50 50 None 0.070 0 230776 Comp. 7 C 1.00 9300 0.515 8.74 50 Pt 0.013 60 130 1425 Comp. 8 B4.00 2700 2.000 8.69 50 None 0.002 0 280 767

[0098] TABLE 2 M_(n) M_(w) 2,1 + 1,3 insertion Mmmm Unsaturated endgroups Example (×10⁻³) (×10⁻³) M_(w)/M_(n) (%) (%) Vin I Allyl 4-Butenylc/t-2-Butenyl Vin II/chain 1 46.4 86.6 1.9 0 83.8 1.00 0 0 0 3.12 2 34.486.1 2.5 0 84.3 0.92 0.08 0 0 2.63 3 43.2 85.4 2.0 0 83.9 1.00 0 0 02.96 4 0 83.5 0.87 0.13 0 0 3.41  5* 40.8 75.6 1.9 1.00 0 0 0 3.02 Comp6 42.0 88.1 2.1 0 84.2 1.00 0 0 0 0.74 Comp 7 19.6 39.2 2.0 0.65 84.60.3 0.04 0.11 0.55 0.20 Comp 8 12.0 27.8 2.3 1.03 85.7 0.73 0.02 0.040.21 0.17

1. A propylene polymer, optionally containing up to S mol % of ethylene,having the following characteristics: i) molecular weight distribution(M_(W)/M_(n)) ≦4; ii) number of internal vinylidene per polymer chain≧2.
 2. The propylene polymer according to claim 1 wherein the molecularweight distribution (M_(W)/M_(n)) is ≦3;
 3. The propylene polymeraccording to claims 1 or 2 wherein the number of internal vinylidene perpolymer chain is ≧2.5.
 4. The propylene polymer according to any ofclaims 1 to 3 wherein the isotactic pentads (mmmm), as determined by¹³C-N analyses, are ≧80%.
 5. The propylene polymer according to any ofclaims 1 to 4 wherein less than 0.5% of the CH₂ groups in the polymericchain are in sequences (CH₂)_(n), wherein n is an even number.
 6. Aprocess for preparing propylene polymer according to any of claims 1 to5, comprising contacting, under polymerization conditions, propylene andoptionally ethylene, with a catalyst system comprising: a) a metallocenecomplex of formula (I)

 wherein: M is titanium, zirconium or hafnium; the groups X equal to ordifferent from each other, are monoanionic sigma ligands selected fromthe group consisting of hydrogen, halogen, —R, —OR, —OCOR, —OSO₂CF₃,—SR, —NR₂ and —PR₂, wherein R is a linear or branched C₁-C₂₀ alkyl,C₂-C₂₀ alkenyl, C₃-C₂₀ cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀ alkylaryl orC₇-C₂₀ arylalkyl radical; the groups R¹, R², R³ and R⁴, equal to ordifferent from each other, are selected from the group consisting ofhydrogen, linear or branched C₁-C₂₀ alkyl C₂-C₂₀ alkenyl, C₃-C₂₀cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀ alkylaryl and C₇-C₂₀ arylalkyl radicals,optionally containing heteroatoms belonging to groups 13-17 of thePeriodic Table; two or four adjacent groups R¹, R², R³ and R⁴ optionallyform together one or more 3-6 membered aromatic or aliphatic rings,optionally substituted with hydrocarbyl radicals optionally containingheteroatoms belonging to groups 13-17 of the Periodic Table; with theproviso that either R¹ is different from R⁴ or R² is different from R³;Z is a carbon or silicon atom; the groups R¹ and R⁶, equal to ordifferent from each other, are selected from the group consisting ofhydrogen, linear or branched, C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀cycloalkyl, C₆-C₂₀ aryl, C₇-C₂₀ alkylaryl and C₇-C₂₀ arylalkyl radicals;optionally containing heteroatoms belonging to groups 13-17 of thePeriodic Table; R⁵ and R⁶ optionally form together a 3 to 6 memberedring; and b) a suitable activating cocatalyst; said process beingcharacterized by reducing the concentration of hydrogen formed duringthe polymerization reaction.
 7. The process according to claim 6,wherein in the metallocene complex: M is zirconium; X is halogen orC₁-C₂₀ alkyl; R¹, R², R³ and R⁴ are hydrogen, C₁-C₂₀ alkyl, or R¹ and R²form a six-membered aromatic or aliphatic ring; Z is a carbon atom; R⁵and Rl are selected from the group consisting of hydrogen, methyl,ethyl, propyl or phenyl;
 8. The process according to claims 6 or 7wherein said metallocene complex has formula

wherein M, X, R³, R⁴, R⁵, R⁶ have the meaning reported in claims 6 or 7and the groups R⁷, R⁸, R⁹ and R¹⁰, equal to or different from eachother, are selected from the group consisting of hydrogen, linear orbranched C₁-C₂₀ all, C₂-C₂₀ alkenyl C₃-C₂₀ cycloalkyl, C₆-C₂₀ aryl,C7C₂₀ alkylaryl and C₇-C₂₀ arylalkyl radicals, optionally containingheteroatoms belonging to groups 13-17 of the Periodic Table.
 9. Theprocess according to claim 8, wherein, in the metallocene complex offormula (I), R³ is a group SiR₃ or CR₃, wherein R has the meaningreported in claim 6, R⁴ is hydrogen and R⁷, R⁸, R⁹ and R¹⁰ are hydrogen,C₁-C₂₀ alkyl or C₆-C₂₀ aryl;
 10. The process according to claim 9,wherein, in the metallocene complex of formula (II), R³ is Si(CH₃)₃ orC(CH₃)₃.
 11. The process according to any of claims 6 to 10 wherein thesuitable activating cocatalyst is selected from alumoxane or compoundscapable of forming an alkyl metallocene cation.
 12. The processaccording to any of claims 6 to 11, wherein the concentration ofhydrogen formed during the polymerization reaction is reduced by meansof a hydrogenation catalyst in the gas phase.
 13. The process accordingto any of claims 6 to 11, wherein the concentration of hydrogen formedduring the polymerization reaction is reduced by means of ahydrogenation catalyst in the liquid phase.
 14. The process according toany of claims 6 to 11, wherein the concentration of hydrogen formedduring the polymerization reaction is reduced by physically removinghydrogen from the gas phase.
 15. The process according to claim 12,wherein the hydrogenation catalyst is a platinum- or a palladium-basedcomposition.
 16. The process according to claim 15 wherein thehydrogenation catalyst is platinum or palladium on alumina.
 17. Theprocess according to claim 13 wherein the hydrogenation catalyst isselected from the group consisting of cobalt- and nickel-basedcatalysts, rhodium and ruthenium catalysts, heterogeneous platinum,platinum oxide and palladium catalysts used as a suspension in thereaction medium.
 18. The process according to any of claims 6 to 17wherein the hydrogen concentration during the polymerization reaction isreduced to less than 50% of the hydrogen concentration in the absence ofremoving means.
 19. The process according to claim 18 wherein thehydrogen concentration during the polymerization reaction is reduced toless than 30% of the hydrogen concentration in the absence of removingmeans.
 20. The process according to claim 19 wherein the hydrogenconcentration during the polymerization reaction is reduced to less than20% of the hydrogen concentration in the absence of removing means.